The aim of this proposes research is to examine the physiologic function of cyclic GMP (cGMP) in the cumulus oocyte complex of graafian follicles during proestrus. Studies by this investigatoar and others show that follicular cGMP levels are highest when gonadotropin values are low. During proestrus, the surge of LH initiates both oocyte maturation and a decline in cGMP in vitro can inhibit occyte maturation only when gonadotropins are absent and when the egg is surrounded by cumulus cells. These studies and the work of others have led me to postulate that during proestrus, cGMP may be acting via the cumulus oophorus to prevent oocyte maturation in preovulatory follicles. The rise in LH causes a decline in cGMP which permits maturation of the oocyte to occur. Though cGMP appears to p lay an inhibitory role in oocyte maturation, it is probably not the sole factor involved in this process. Cyclic nucleotide and steroid levels will be measured by radioimmunoassay in whole graafian follicles at various stages of oocyte maturation. These studies will be performed using follicles from normal proestrous hamsters and in vitro models. Both transmission and scanning electron microscopy will be u tilized to examine the utrastructual aspects of oocyte maturation influenced by maturation inhibitors and gonadotropins. The proposed studies introduce, for the first time, a role for cGMP in oocyte maturation. The experiemts presented in this grant will attempt to examine further the effect of cGMP on maturation of the mammalian oocyte.